Since scandium has an especially small ionic radius of rare earth elements, the scandium exists hardly in usual rare earth elements, but occur widely with a very small amount in oxide ores, aluminium, tin, tungsten, zirconium, iron, and nickel or the like.
The scandium has low basicity with the small ionic radius, and thus needs a strong acid for dissolution thereof. Accordingly, because there are so many coexistent elements to be simultaneously dissolved with the scandium and concentration of an aqueous solution is high, it is not easy work to separate and refine the scandium contained in the aqueous solution.
As a typical separation method, the solvent extraction method is widely known, in which an organic solvent is used as an extractant composed of acidic alkyl phosphoric acid, a product name, PC-88A, (principal ingredients: 2-ethylhexyl phosphonic acid 2-ethylhexyl) or the like, as disclosed, for example, in Patent Document 1. Specifically, the solvent extraction method is a method in which an organic solvent is mixed with an aqueous solution containing scandium to extract coexistent elements, iron, aluminium, calcium, and yttrium or the like with the scandium into the organic solvent. Then, a hydrochloric acid solution having concentration of 4 to 9 mol/l is added to the organic solvent to thereby clean the coexistent elements for separating and removing the elements other than the scandium. Finally, a sodium hydroxide aqueous solution is added to the organic solution to separate the scandium from the organic solvent in the form of hydroxide.
However, the method disclosed in Patent Document 1 is unable to strip the scandium as an aqueous solution, and therefore scandium hydroxide is separated from the organic solvent, as solid gummy or gelatinous precipitations. For this reason, it becomes difficult to make a solid-liquid separation between the solvent and the scandium hydroxide, and a problem emerges therefrom that the scandium hydroxide is contaminated by solvent, specially, by phosphorus. Further, since a large amount of extractant is dissolved into water, as sodium salts, every time the scandium is stripped, it is also needed to comprise a recovery treatment of the extractant from an aqueous phase after stripping. Failure to the recovery treatment will result in incurring a markedly increased COD value of drainage, and causing an environmental problem at the end.
Moreover, when zirconium is contained in a solution containing the scandium, a problem emerges therefrom that a large amount of zirconium contained in the scandium produced by the solvent extraction method induces the degradation of the the scandium, as they share a close extraction characteristics of the scandium with each other.
Besides, in an industrial aspect, since the solid gummy or gelatinous precipitations are precipitated and separated, as mentioned above, it cannot proceed with a separation and refinement operation from proceeding by a continuous extraction operation, such as, for example, a mixer settler. Accordingly, it is obliged to proceed a treatment by resorting to a cumbersome batch work, every time the scandium is stripped, and is disadvantageous in terms of labor and cost.
Regarding an extractant, acidic phosphoric ester having an alkylcyclohexyl group has also been invented, as an alkyl chain, in place of a normal 2-ethylhexyl group (for example, see Patent Document 2). However, while such an extractant exhibits a change in a separation factor to impurity elements caused by steric hindrance, the above-indicated problem is still left unsolved as a strong chemical bonding force to the scandium remains unchanged.
Meanwhile, it also contrives to use acidic alkyl phosphoric ester, like an adsorbent in the form where the acidic alkyl phosphoric ester is being supported by resin, instead of the solvent extraction method.
For example, there is disclosed, in Patent Document 3, a method in which 2-ethylhexyl phosphonic acid, di(2-ethylhexy)phosphonic acid, and tributyl phosphoric ester or the like are supported by resin, and the resin is developed in an inorganic strong acid aqueous solution, such as hydrochloric acid, nitric acid, and sulfuric acid, or an organic acid aqueous solution, such as acetic acid, and monochloroacetic acid.
For example, there is disclosed, in Patent Document 4, a method in which scandium is adsorbed by resin impregnated with alkyl phosphonic acid ester and alkyl phosphate ester, and thereafter scandium is leached by an organic solvent together with an extractant.
The methods disclosed in Patent Documents 3 and 4, however, are fraught with difficulties in elution of the scandium from the resin, and have properties such that the scandium forms polymer with an extractant on a resin surface, and solidifies thereon, even in a step of adsorbing the scandium. On this account, a problem emerges therefrom that the scandium itself hinders diffusion of the scandium to the whole extractant with the progress of adsorption.
As a solution to the problem in case of using the above-mentioned phosphate ester, it has been known heretofore a method of using lipophilic aminocarboxylic acids, as is disclosed, for example, in Patent Document 5.
Because weak acidic aminocarboxylic acid has weak affinity with the scandium, it has a property that stripping is easier than acidic phosphate ester. Nonetheless, the aminocarboxylic acid conversely has a strong bonding force with chromium (III), iron (III), and ion or the like, having a high complex stability constant with the aminocarboxylic acid, and therefore, in an extracting power of the scandium in a solution containing these impurities, the impurities are accumulated in a solvent, resulting in a gradual decrease. Further, the structurally strong hydrophilicity entails a problem that a leaching loss to an aqueous solution grows larger.
In this way, a method for allowing efficient separation and refinement of the scandium has not yet been found at the moment.